In the field of mass analysis using an ion trap mass spectrometer or other apparatuses, a technique called the MS/MS analysis (or tandem analysis) is conventionally known. In a general MS/MS analysis, an ion having a specific mass (or mass-to-charge ratio, m/z, to be exact) is first selected as a precursor ion from an object to be analyzed. Next, the selected precursor ion is dissociated by a collision induced dissociation (CID) process to produce product ions (also called fragment ions). The resulting product ions are subjected to a mass analysis to obtain information relating to the mass of the product ions, the ions and neutral molecules desorbed by the dissociation operation, and other particles. Based on this information, the composition and chemical structure of the target sample molecule are deduced.
In recent years, samples to be analyzed with this type of system have been progressively increasing in molecular weight and becoming more complex in structure (composition). Therefore, depending on the nature of the sample, it is possible that the sample cannot be dissociated into sufficiently small masses by only one stage of the dissociation process. In such a case, an MSn analysis may be performed, where the dissociation operation is repeated two or more times and the eventually obtained product ions are subjected to mass analysis (for example, refer to Patent Document 1, 2 or other documents). The aforementioned MS/MS analysis is an MSn analysis where n=2.
By the way, analyzing metabolites resulting from chemical changes in a living organism has been a crucial subject in many fields, such as the diagnosis of various kinds of disease and illness, the assessment of the effectiveness and safety of drugs and functional foods, and the research on lifestyle and health. In recent years, a method called Metabolomics for exhaustively analyzing a metabolite has been attracting attention. In this metabolite analysis, when it is necessary to search for a compound resulting from a metabolism of another compound having a known structure (this compound will be hereinafter called a “parent compound”, and the former compound will be called a “metabolite”), the aforementioned method using MSn spectrums collected by subjecting the parent compound having a large molecular weight to one or more stages of the dissociation operation is useful.
In the case of applying an MSn analysis to a metabolite analysis, it is necessary to perform a multi-stage dissociation operation on the site of metabolism specific to the metabolite (i.e. the portion modified by a part of the structure originating from the parent compound) in order to clarify the chemical structure of the metabolite. That is to say, it is important that an appropriate ion among a number of peaks of the product ions obtained by a given stage of the dissociation operation should be selected as the precursor ion for the next stage of the dissociation operation. In one conventional method for automatically selecting the precursor ion, a plurality of peaks with signal intensities equal to or greater than a predetermined threshold are extracted from the peaks appearing on a mass spectrum and subjected to a selection process using a specific criterion (e.g. in the order of decreasing intensities or increasing mass-to-charge ratios). However, this conventional precursor ion selection method does not guarantee that the ion corresponding to the site of metabolism in question will be always selected.
Therefore, in practice, an analysis operator visually checks the MSn spectrum to select an ion appropriate as the next precursor ion or specify the order of priority for selecting the precursor ion. However, making such decisions is rather difficult, and the correctness of the decision inevitably depends on the experience, skill and other factors of the analysis operator.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H10-142196
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-249114